After mastering a musical instrument, what comes next?

It takes time to learn playing a musical instrument. In the process you will always have something to play from someone.
But the problem is after like 15 or 20 years, how do you keep playing? The books are finished. Should we improvise or compose? I think we are somehow trapped inside our minds. How do we get out of it to find new ideas to play?

Apr 28 2012:
If you've mastered your instrument and played it for audiences to enjoy, teaching others what you know best regarding your musical experiences comes next. I believe you can find motivation to keep playing from the growth of your potential music students.

Apr 29 2012:
Thats for your Comment,
I agree. Teaching is very joyful. But the problem is after sometime the whole teaching process makes you a trainer, and then you can just improve you teaching skills not playing skills. I tried concerts too, but soon I got cold feet again.
Do we have any other idea to know more about the music? How to pass the borders of learning?

May 1 2012:
Yesterday I read about an artist .. who wants to take her music to a different level.. She was re searching into
music therapy and she plays at hospitals and has observed that some "raagas" help heal ....

Apr 29 2012:
A second one, and a third, and a fourth, then you start getting very creative and play one as if it were the other, you find new ways to play those instruments, then you make some youtube videos and we all enjoy your performances!

Apr 30 2012:
gosh, suppose you are tone deaf of something. My experience. move on to something else besides
music. Of course, just fun is not like classical music training. If you know music and are just awful or
bored. You can play a piano from a fake book, play azithe, play a flute, or try to play a kazoo. gosh.
if you are like me and have no rhythm. don't highlight it. I have a funny story from the U>S> Marine Corps
about me. Of ourse, Carl Jung does write about this common problem. Read Strength finder, whidh
is ab interesting book.